Hello Maxschle,
Welcome to the VMware communities forum.
While you can as stated in the other answer use Bootcamp, bootcamp does of course mean "rebooting" and running windows only and rebooting again if you want to use OS X.
If you want to enjoy both operating systems (OS X and windows) at the same time, then virtualisation is the answer.
In regards to "growing the virtual harddisk". You can do that when you are starting to run out of space in your windows guest OS.
It is MUCH easier to grow your disk as to shrink it and as such I do recommend to not make it "as big as you can".
In order to grow the disk in your guest OS, shut it down completely (not pause/resume/..) using the shutdown feature in windows.
Then go into Fusion menu "Virtual Machine", "Settings", "Hard Disk" and increase the size.
On starting your guest OS, the virtual disk will be bigger.
Hope this helps,
--
Wil